Asphalts are well known and widely used in a variety of products. While asphalts are primarily composed of high molecular weight hydrocarbons, they invariably contain minor amounts of low molecular weight hydrocarbons exhibiting substantial volatility. As such, the manufacture, storage and transportation of asphalt materials present opportunities for escape of such volatile, organic components (VOCs) into the atmosphere, as well as the accumulation of highly explosive vapors in storage facilities and processing equipment.
In view of the environmental and safety hazards such VOCs present, methods to control their accumulation and emission have been developed. Typically, these involve drawing the vapors into a flame incinerator where they are combusted. Unfortunately, the incineration of such VOCs by a flame burner is expensive, and the temperatures reached in such incinerators often favor the formation of undesirable nitrous oxides.
Although regenerative thermal oxidizers (RTOs) are well known and used in various industries for the treatment of effluent gas streams to reduce VOCs, their usefulness in conjunction with the manufacture, storage and transportation of asphalt has not previously been fully appreciated due, at least in part, to the failure of commercially available RTO devices to accommodate the relatively high content of condensable vapors or blowing distillate oil (BD oil) entrained in the gas stream drawn off from such operations. When asphalt vapors are introduced directly into an RTO unit, the BD oil contained in such streams quickly forms a layer of coke on the heat transfer elements of the RTO unit which reduces its effectiveness. As such, the frequent cleaning and/or replacement of the heat transfer elements in such units has previously rendered their use in conjunction with the manufacture of asphalt uneconomical. Although advances in this regard have been made by incorporating a cyclone separator upstream of the RTO unit to remove BD oils, such as in the BIOTOX systems commercially available from BIOTHERMICA, the performance of such separators at low or varying flow rates, or for removing particles of varying sizes, is less than optimum. Accordingly, a need exists for a RTO system capable of effectively removing the condensable oils and oxidizing the VOC's contained in asphalt vapors over a wide range of particle sizes and flow rates. This need is met by the invention described herein.